The aim of this project is to study the role of social housing group composition (presence/absence of sire, proportion of pregnant females in group, group size and stability) on various measures of reproductive outcome. In the retrospective stage of the study we analyzed the records of 4803 pregnancies in pigtailed macaques; a report of our findings has been accepted for publication in the American Journal of Primatology. During the past year we have continued this research using both a records-monitoring approach and a hands-on experimental approach. We have been monitoring animals in the Seattle breeding colony for estrus cycle lengths, pregnancy outcome, interbirth intervals, and relative levels of social aggression. The presence of adult males has been manipulated as part of normal husbandry requirements. In the coming months we will analyze the results and prepare them for publication.